The practice of electronically testing industrially fabricated printed circuit boards has gained wide acceptance. Thus, the quality of the boards may be judged prior to further processing, such as the installation of comparitively expensive electronic components on the boards. If a printed circuit board has one thousand contacts, about 500,000 individual tests must be conducted on two-point connections to ensure that the board has been reliably tested. It has been found that the overall testing cannot be performed economically and in a justifiable time period unless it is done electronically by means of a computer.
The computer-controlled printed circuit board testing apparatus of this kind is programmed by placing a faultless master circuit board in a mount provided to receive the printed circuit boards. Then all possible combinations of two-point connections are automatically tested and the computer memorizes the results of each possible connection on the master circuit board. Thereafter, production circuit boards are fed into such apparatus for testing. In the testing process, each individual two-point connection is compared with the memorized information, and in the absence of identity between the measured information and the information memorized, an error output is generated and, if desired, the defective circuit board is automatically ejected.
The testing apparatus requires one contact for each possible connection point on each circuit board to be tested by the circuit board testing apparatus, with all these contacts arranged in a so-called contact array or reference grid. German Laid-Open Application No. 31 10 056, which is the starting point for the present invention, and the disclosure of which hereby is incorporated herein by reference, teaches a contact assembly for circuit board testing apparatus including a plurality of contacts provided in a contact plane. These contacts are arranged in groups of eight along the end surfaces of elongated flat contact plates shaped substantially like the letter "I".
These plates are held one behind the other in a dense pack by means of horizontal supports and form a strip of the contact plane which has a width of eight contacts. The horizontal supports receive the contact pressure, of which the total may assume considerable levels. The contact array or base grid in its entirety is made up of a plurality of such strips placed side by side, with the total array or grid possibly including many thousands of individual contacts. The purpose of dividing the contact array into a great number of plates or segments of this kind is to enable one or several of these segments to be removed as may be needed without having to disassemble the entire array.
At the other ends remote from the contact array, the I-shaped contact plates having the contacts at their one end are connected to wires for coupling to the computer-controlled test circuitry of the circuit board testing apparatus. Each contact has one wire connected thereto. As a result, a vast number of wires must be run along the bottom side of the contact array, presenting major difficulties to the repair or replacement of individual contact plates. Also, as these wires must be run up to the areas of the contact array in which they are connected, modifications of the contact array (for adaptation to the contours of a circuit board to be tested, for example) are not provided for or even possible in practical applications. Also, the circuit board testing apparatus must be equipped from the start with a complete complement of the required testing electronics so that it will be able to test the maximum possible number of contacts on a circuit board although this extreme case may never occur.
It is an object of the present invention to provide a contact assembly for printed circuit board testing apparatus such that the user without difficulties may set up the contact array as to both the number of the required contacts and their distribution pattern across a given area within some given outer boundaries. In particular, the user is to be placed in a position merely having to add, in steps, the electronic circuitry needed for testing printed circuit boards having an increasing number of contacts.
Thus, the user's printed circuit board testing apparatus need only be equipped with precisely the amount of relatively expensive electronics which is actually needed to test the printed circuit boards of interest to the user. In other words, the invention seeks to make possible a printed circuit board testing apparatus which has a contact array expandable as needed in steps from a minimum of active contacts to a maximum possible number thereof and which, moreover, may be adapted to the shape of any specific circuit board to be tested within the limits determined by the outer dimensions of the apparatus.